Hauenstein Mareike, Christ Bastien, Das Aditi, Aubry Sylvain, Hörtensteiner Stefan
Institute of Plant Biology, University of Zurich, CH-8008 Zurich, Switzerland.
Institute of Plant Biology, University of Zurich, CH-8008 Zurich, Switzerland
Plant Cell. 2016 Oct;28(10):2510-2527. doi: 10.1105/tpc.16.00630. Epub 2016 Sep 21.
Chlorophyll degradation is the most obvious hallmark of leaf senescence. Phyllobilins, linear tetrapyrroles that are derived from opening of the chlorin macrocycle by the Rieske-type oxygenase PHEOPHORBIDE a OXYGENASE (PAO), are the end products of chlorophyll degradation. Phyllobilins carry defined modifications at several peripheral positions within the tetrapyrrole backbone. While most of these modifications are species-specific, hydroxylation at the C3 position is commonly found in all species analyzed to date. We demonstrate that this hydroxylation occurs in senescent chloroplasts of Arabidopsis thaliana. Using bell pepper (Capsicum annuum) chromoplasts, we establish that phyllobilin hydroxylation is catalyzed by a membrane-bound, molecular oxygen-dependent, and ferredoxin-dependent activity. As these features resemble the requirements of PAO, we considered membrane-bound Rieske-type oxygenases as potential candidates. Analysis of mutants of the two Arabidopsis Rieske-type oxygenases (besides PAO) uncovered that phyllobilin hydroxylation depends on TRANSLOCON AT THE INNER CHLOROPLAST ENVELOPE55 (TIC55). Our work demonstrates a catalytic activity for TIC55, which in the past has been considered as a redox sensor of protein import into plastids. Given the wide evolutionary distribution of both PAO and TIC55, we consider that chlorophyll degradation likely coevolved with land plants.
叶绿素降解是叶片衰老最明显的标志。叶胆素是一类线性四吡咯，由 Rieske 型加氧酶脱镁叶绿酸 a 加氧酶（PAO）打开二氢卟吩大环而产生，是叶绿素降解的终产物。叶胆素在四吡咯骨架内的几个外围位置带有特定修饰。虽然这些修饰大多具有物种特异性，但 C3 位的羟基化在迄今分析的所有物种中普遍存在。我们证明这种羟基化发生在拟南芥衰老的叶绿体中。利用甜椒（辣椒）的有色体，我们确定叶胆素羟基化是由一种膜结合的、依赖分子氧和铁氧还蛋白的活性催化的。由于这些特征类似于 PAO 的要求，我们认为膜结合的 Rieske 型加氧酶是潜在的候选者。对拟南芥两种 Rieske 型加氧酶（除 PAO 外）的突变体分析发现，叶胆素羟基化依赖于叶绿体内膜转位因子 55（TIC55）。我们的工作证明了 TIC55 的催化活性，过去它被认为是蛋白质导入质体的氧化还原传感器。鉴于 PAO 和 TIC55 在进化上的广泛分布，我们认为叶绿素降解可能与陆地植物共同进化。
